Information transmission by means of radio wave propagation has become ubiquitous since Marconi's demonstration of radio-telegraphy in the early 1900's. Until the early 1960's, information was generally transmitted over radio communications circuits by means of analog modulation of a carrier frequency. Transmission of digital data evolved from the late 1950's onwards, in the beginning to support the development of military communications equipment. Digital data transmission was initially motivated by Claude Shannon's seminal investigations of channel capacity. Since then, this technology has been applied to support widespread commercial use. In all of these applications, the means to provide two-way communications require both the sender and the receiver to be equipped with a combination of radio transmitting and receiving equipment, i.e. with a transceiver.
Currently, any person who operates the Global Positioning System (GPS) user equipment must provide a separate communications transceiver, in order to conduct two-way communications of data messages while using GPS equipment for navigational purposes. As well known, the GPS is a network of at least 24 NAVSTAR satellites, which orbit the earth at an altitude of about 11,000 miles. GPS user equipment can locate the position of a user anywhere on earth by very accurately measuring the distance between the user and at least four GPS satellites. The GPS satellites provide specially coded signals that are processed in a GPS receiver, enabling that receiver to generate the user's position, velocity and time by computing the relative transit times of signals transmitted simultaneously from a multiplicity of the GPS satellites. In this way, the GPS provides precise positioning and timing data, which are useful for a variety of applications, including, but not limited to, navigation, surveillance, mapping, and vehicle tracking.
At present, however, an operator of GPS equipment must procure a separate and independent radio communications system in order to engage in two-way radio communications. The operator must thus bear the significantly increased cost, volume, weight and power consumption associated with providing separate and independent radio communications systems.
Accordingly, it is desirable to provide a more efficient and inexpensive method and system for enabling two-way (bi-directional) radio communications of data between two or more users who operate GPS navigation user equipment. In particular, it is desirable to provide both communication and GPS navigation services within a single GPS transceiver. In this way, the cost, volume, weight, and power consumption that are necessary in order to provide both communications and navigation services can be significantly reduced.